Imaging systems that detect various electromagnetic radiation spectral bands, including the visible band, and throughout the infrared band, can be used in a variety of applications, such as detection of chemical or biological agents, and anomaly and threat detection. Another application is the ability to see a target in a cluttered environment. One type of image system may use a focal plane array (FPA) to detect infrared radiation. An FPA may be formed from an array of detector elements, such as infrared detector elements, wherein each of the infrared detector elements functions as a pixel to produce an image of the detected items. The change in signal level output of each of the infrared detector elements due to interaction with incident infrared radiation is converted into an electrical signal by a read-out integrated circuit (ROIC).
An infrared image typically provides a grayscale map of objects in a scene. In multi-spectral imaging (MSI), multiple images of a scene or object are created using radiation from different parts of the spectrum. The reflection characteristics of an object in a scene are dependent on the wavelength of light that is being detected. Hence, multi-spectral imagery provides added contrast when compared to a broad-band image.
Broadband detectors, also referred to as single-color detectors that operate in the visible/near-infrared/short-wave infrared (VIS/NIR/SWIR, or VNS) band (0.4 to 2.5 μm), can be fabricated from HgCdTe or InGaAs-based semiconductor materials. Broadband detectors are fabricated from a two-terminal photodiode.
Multi-spectral imagers resolve incident radiation into different wavelength bands, on bin, and send the binned signals back to the ROIC. Multi-spectral detectors are in general more complicated compared to the broadband counterparts because of the need for electrical interconnects to the different absorber elements in the detector. Multi-spectral detectors capable of detecting three or more bands typically require at least three terminals per pixel to operate the device.
InGaAs-based sensors are used widely for night vision and other SWIR imaging applications. However, these sensors are not capable of multi-band sensing and hence provide gray-scale images that can be difficult to comprehend. The use of three bands provides added contrast for target identification and discrimination.
What is needed is a two-terminal detector capable of detecting a plurality of bands, such as three bands. It is desirable for a detector to be capable of detecting at least three bands over the visible and short-wave infrared (0.4-2.5 micron) bands. Detection of three bands enables one to construct composite-color imagery that provide significantly added contract in comparison to typical grayscale images.
Furthermore, it is desirable for a detector to operate at ambient temperature or require minimal cooling. There is a commercial need for such a detector that is simple, lightweight, and efficient. It would be further beneficial for such a detector to be suitable as a drop-in upgrade to conventional single-color sensors. The invention described herein can fulfill these needs.